In various industries such as the automobile industry, the shipbuilding industry, and the aerospace industry, the friction stir welding method, which is a solid-phase joining method, has been widely used, by virtue of its advantage that deformation or cracking caused by thermal strain is less likely to arise and the joining strength increases, as compared with the case of a melt welding process, such as TIG or MIG.
Incidentally, the friction stir welding method uses a friction stir welding tool which includes a cylindrical columnar rotor attached to a rotation-drive section, and a probe concentrically provided on a distal end surface of the rotor. The probe is plunged into a joint interface between two workpieces while being rotated, and the probe and the two workpieces are caused to undergo relative movement. Therefore, friction heat and relatively large pressure are applied to the workpieces. Accordingly, the workpieces move and deform at the time of joining, possibly generating joining defects, such as an internal defect of a joined portion and a form defect (e.g., misalignment). In order to prevent generation of such joining defects, the workpieces must be rigidly fixed during the joining.
However, a considerable difficulty arises when a base having a circular recess and a circular cover fitted into the recess and closing the opening of the recess are joined together by friction stir welding. Specifically, for such joining, the base having a circular recess is prepared along with the cover to be fitted into the recess and to close the opening of the recess. A stepped portion for receiving a circumferential edge portion of the cover is formed on the inner circumferential surface of the recess at an intermediate depth thereof. After the cover is fitted into the recess such that the circumferential edge portion thereof rests on the stepped portion of the recess, the base and the cover are joined together by means of the friction stir welding along a joint interface between a circumferential edge portion of the opening of the recess of the base and the circumferential edge portion of the cover. However, in such a case, it is extremely difficult to carry out the friction stir welding over the entire joint interface in a state where the base and the cover are firmly fixed.
In order to cope with the above problem, an improved method has been proposed (see Patent Document 1). In this method, when a cover is to be friction-stir-welded to a base as described above, the cover is provisionally and partially joined to the base by a general melt welding process or a friction stir welding process.
However, in the case where the cover is provisionally joined by a melt welding process according to the method disclosed in the Patent Document 1, a completely different apparatus must be used for such provisional joining, and the number of steps increases. Therefore, the work of joining the cover to the base is troublesome. Further, in the case where the cover is provisionally joined to the base at several points by a melt welding process, a blowhole, cracking, poor fusion, or a like defect is likely to arise. In order to avoid occurrence of such defect, preflow, treatment of craters, and the like must be sufficiently performed before main joining. This leads to an increase in the number of steps, thus impairing mass productivity.
Meanwhile, in the case where the cover is provisionally joined by a friction stir welding process, such provisional joining must be performed by use of a probe which is smaller in diameter than a probe used for main friction stir welding. However, for such provisional joining, the probe for provisional joining must be attached to the rotation-drive section of a friction stir welding apparatus so as to be replaced with the probe for main joining. Such probe exchange work is troublesome.
The present applicant has proposed a friction stir welding tool in which the probe is composed of a plurality of shaft portions having different diameters such that the diameter of the probe decreases stepwise toward the distal end thereof (see Patent Document 2). However, the friction stir welding tool disclosed in Patent Document 2 is designed to enable joining of a plurality of workpieces which differ in thickness at respective portions to be joined, and to secure a stable and high joining quality even when thick members are welded together. Therefore, the diameter ratio between the small diameter portion and the large diameter portion of the probe is not optimal for carrying out both the provisional friction stir welding and the main friction stir welding.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2002-248584
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2000-246465